Grinding wheel

ABSTRACT

A grinding wheel which includes abrasive grits characterized in that at least part of the abrasive grits are comprised of at least about 0.5% by weight of at least one modifying metal oxide and wherein the concentration of said modifying metal oxide is greater at or near the surface than at the interior of said grit.

REFERENCE TO PRIOR APPLICATIONS

This is a continuation of application Ser. No. 07/976,893 filed Nov. 16,1992, now U.S. Pat. No. 5,312,789, which is a continuation ofapplication Ser. No. 07/799,867 filed Nov. 27, 1991, now U.S. Pat. No.5,164,348, which is a continuation of application Ser. No. 07/645,349,filed Jan. 24, 1991, abandoned, which is a continuation-in-part ofapplication Ser. No. 07/054,440, filed May 27, 1987, abandoned.

TECHNICAL FIELD

This invention relates to the production of alumina-based ceramicabrasive grits, a method of making the same, abrasive products made withthe abrasive grits and a method of using the abrasive products.

BACKGROUND ART

The preparation by a sol-gel process of dense, alumina-based ceramicabrasive grain is described, for example, in Leitheiser et al, U.S. Pat.No. 4,314,827, assigned to the assignee of the present application. Thispatent teaches making abrasive grains by employing chemical ceramictechnology by gelling alumina monohydrate with a precursor of at leastone modifying component followed by dehydration and firing. Themodifying component is selected from zirconia, hafnia, a combination ofzirconia and hafnia, and a spinel derived from alumina and at least oneoxide of cobalt, nickel, zinc, or magnesium.

Abrasive products containing ceramic abrasive grits made by a sol gelprocess have been found to perform in a superior manner as compared tothe best fused synthetic abrasive mineral in many applications. Atypical example of a high performance fused synthetic abrasive mineralis formed of fused alumina-zirconia available, for example, under thetrade designation "NorZon" from the Norton Company.

Other references which disclose the preparation of alumina-based ceramicabrasive grains include the following:

Cottringer et al, U.S. Pat. No. 4,623,364, issued Nov. 18, 1986,entitled Abrasive Material and Method For Preparing The Same.

Gerk, U.S. Pat. No. 4,574,003, issued Mar. 4, 1986, entitled Process ForImproved Densification of Sol-Gel Produced Alumina-Based Ceramics.

Amero, U.S. Pat. No. 3,450,515 discloses a method of making impregnatedsintered bauxite abrasive grains. The grains are prepared byimpregnating sized particles of calcined bauxite with an aqueoussolution of manganese, iron or copper ions, and firing the impregnatedparticles for at least two hours at 1600° C. The resultant abrasivegrains are said to contain agglomerated alpha alumina crystals of a sizebetween 50 and 200 microns, with the preferred crystal size of about 100microns and higher being preferred. Bauxite is an impure form of aluminacontaining other oxides including, for example, iron oxide, titania, andsilica. Instead of producing abrasive grains of higher strength, theresultant grains are said to be weaker, a result which the patenteeappears to desire for stainless steel snagging.

References which disclose the preparation of ceramics include thefollowing:

Church et al, U.S. Pat. No. 4,007,020 discloses a method of producing arefractory abrasive body by forming a porous skeletal body which isimpregnated with a compound of a metal capable of being converted to anoxide in situ at relatively low temperatures, heating the body soimpregnated at a temperature well below the normal vitrification to atemperature of at least 600° F. and for a time sufficient to convert thecompound impregnated therein to an oxide and repeating the impregnationand heating steps until the desired degree of hardness is obtained. Theporous body can be made from relatively finely divided materials whichmay be relatively pure powders, mixtures of powders, or impure powders,including additives in the form of discrete particles, fibers, fillersand the like. The powders are molded and bound together or boundtogether and molded with a binder which may comprise the impregnatingcompound or other suitable binder prior to treatment. Such refractorymaterials include alumina, beryllia, magnesia, titania, and zirconia.

Berneberg et al., U.S. Pat. No. 4,552,786, discloses a method fordensification ceramic materials which involves dissolving a ceramicprecursor in a supercritical fluid, infiltrating the low density ceramicmaterial with the ceramic precursor-laden fluid, and reducing thesolubility of the ceramic precursor in the fluid to impregnate theceramic precursor in the void spaces of the ceramic material.

Bailey et al., U.S. Pat. No. 3,859,399, discloses a method for makingdense composite ceramic bodies of titanium diboride, boron carbide,silicon carbide and silicon. The ceramic bodies are produced by forminga mixture of titanium diboride, boron carbide and a temporary binderinto a desired shape to obtain a coherent green body which issiliconized by heating in contact with silicon to a temperature aboutthe melting point of silicon, where upon the molten silicon infiltratesthe body and reacts with some of the boron carbide therein to producesilicon carbide in situ.

Bugosh, U.S. Pat. No. 3,108,888, discloses processes for producingcolloidal, anisodiametric transition aluminas by heating colloidal,anisodiametric boehmite at a temperature in the range of 300° to 1000°C. until the desired conversion has occurred, and is further directed tothe process for making strong, shaped bodies by forming a mass of suchboehmite particles and so heating until the boehmite is converted into atransition alumina, and optionally to the alpha form. The temperature ofheating could be above the sintering point. Another-method involves theintroduction of grain-growth inhibitors and/or sintering-promotingsubstances such as iron oxide, manganese oxide, copper oxide andtitanium oxide to impregnate a porous object of gamma alumina derivedfrom fibrous colloidal boehmite alumina powder. Impregnation by aqueoussolutions of soluble precursors of the desired oxide modifier aresuggested, followed by drying to fix the modifier on the surface of theindividual gamma alumina powders. This patent does not, however, teachthe preparation of abrasive grits by this method.

SUMMARY OF THE INVENTION

The present invention provides hard, durable ceramic abrasive gritsformed by an impregnation process which have superior abrasiveperformance in abrading certain workpieces such as those made ofstainless steel, titanium, high nickel alloys, aluminum and others, andexcellent performance on more conventional workpieces such as mildsteel. The ceramic abrasive grits comprise alpha alumina and at leastabout 0.5% (preferably about 1% to about 30%) by weight of the oxide ofat least one metal, as a modifying additive. The additive metal isselected to provide upon calcining and firing, a durable abrasive grit.Preferred additive metals are selected from zirconium, magnesium,hafnium, cobalt, nickel, zinc, yttrium, praseodymium, samarium,ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, anderbium. The preferred modifying additive is an oxide of yttrium,magnesium, and a rare earth metal selected from praseodymium, samarium,ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, anderbium.

The additive metal oxide will usually react with aluminum oxide to forman additive metal-aluminum oxide reaction product. The oxide of thereaction product of cobalt, nickel, zinc and magnesium will generally bea spinel. The oxide of the reaction product dysprosium and gadoliniumwith aluminum oxide will generally be a garnet, while the oxide of thereaction product of praseodymium, ytterbium, erbium and samarium withaluminum oxide will generally be a perovskite which may include somegarnet. The reaction between lanthanum and aluminum oxides results inbeta alumina. The alumina in each case which has not reacted with theadditive metal oxide will be substantially in the alpha form.

The general process of making the ceramic abrasive grits of theinvention comprises the steps of:

(1) preparing a sol of alumina hydrate, preferably the monohydrate(e.g., boehmite);

(2) drying the sol to form a porous solid comprised of the dried sol;

(3) crushing the dried solid to produce particles;

(4) preparing a homogeneous mixture of an additive metal oxide or itsprecursor, preferably a water soluble salt, in a liquid vehicle such aswater;

(5) calcining the dried particles to substantially remove water ofhydration and convert the alumina hydrate to an alumina form which isinsoluble in the liquid vehicle;

(6) impregnating the mixture of step (4) into the calcined particles toachieve an average concentration of metal oxide in the resultant ceramicof at least about 0.5% by weight upon firing to produce a ceramic;

(7) drying the impregnated particles;

(8) calcining the dried impregnated particles to substantially removebound volatile material; and

(9) firing the particles to produce ceramic abrasive grits.

Each of the abrasive grits in the same batch made by the methoddescribed above is characterized by having a substantially uniformexterior portion from grit to grit comprised of additive metal oxide andalumina. By contrast, abrasive grits made by mixing a solution of a saltof the additive metal with an alumina hydrate hydrosol, gelling themixture, drying the gel, crushing the dried gel to produce particles,calcining the particles, and firing the calcined particles, as forexample described in assignee's U.S. Pat. No. 4,314,827, will have lessuniformity from grit to grit. This is thought to be due to migration ofthe ionic additive metal ion as the gel is dried, which causes aconcentration of the additive metal at the surface of large dried gelchunks. Crushing the large surface-enriched, dried gel chunks producesparticles which originate from various parts of each dried gel chunk,some from its interior which would be deficient in additive metal, somefrom the additive metal-enriched surface, and some from the transitionarea of the dried gel chunk which would produce a particle having onlypart of its surface enriched with the additive metal. The presentinvention avoids the problem of grit to grit nonuniformity by preparinggrit sized particles of dried alumina which are calcined and then eachis impregnated with the additive metal usually as a solution, dried,again calcined, and fired to produce abrasive grits.

Abrasive grits which are deficient in additive metal are characterizedby being porous while abrasive grits which have an adequate amount ofadditive metal are substantially non-porous and dense. The difference inabrasive grains made by the presently claimed method and those of theprior art described above may be demonstrated by immersing grains ofeach method in a colored dye solution. The porous abrasive grits willabsorb much larger amounts of the dye than the dense abrasive grits. Ofcourse, dense abrasive grits perform better than porous abrasive grits.

The invention also provides various abrasive products of a type whichincludes abrasive grits characterized in that at least part of theabrasive grits are the ceramic abrasive grits of the invention.Preferred abrasive articles include coated abrasive products, bondedabrasive products such as grinding wheels, and nonwoven abrasiveproducts in a form similar to that sold under trade designationScotchbrite® by the 3M Company.

DETAILED DESCRIPTION

The preparation of the ceramic abrasive grits from an impregnationprocess first includes the preparation of a dispersion of aluminum oxidemonohydrate (e.g., boehmite), usually comprising about 10 to about 60weight percent aluminum oxide monohydrate.

The boehmite can either be prepared from various techniques well knownin the art or can be acquired commercially from a number of suppliers.Examples of commercially available materials include that availableunder the trade designation Disperal® produced by Condea Chemie, GMBHand that available under the trade designation Catapal® S.B., producedby Vista Chemical Company. These aluminum oxide monohydrates aretypically in the alphaform, are relatively pure, include relativelylittle, if any, hydrate phases other than the monohydrate, and have ahigh surface area.

A peptizing agent is usually added to the boehmite dispersion to producea more stable hydrosol or colloidal dispersion. Monoprotic acids or acidcompounds which may be used as the peptizing agent include hydrochloric,acetic, and nitric acid. Nitric acid is a preferred peptizing agentmultiprotic acids are normally avoided since they rapidly gel thedispersion, making it difficult to handle or mix with additionalcomponents. Some commercial sources of boehmite contain acid titer, suchas absorbed formic or nitric acid, to assist in forming a stabledispersion.

The dispersion may contain a nucleating agent to enhance thetransformation to alpha alumina. Suitable nucleating agents include fineparticles of alpha alumina, alpha ferric oxide or its percursor and anyother material which will nucleate the transformation. The amount ofnucleating agent is sufficient to effect nucleation. Nucleating suchdispersions is disclosed in assignee's European patent application No. 0200 487, published Nov. 5, 1986.

The dispersion is then dried by conventional means to form a poroussolid. Drying may be accomplished in steps, first forming a plastic massof the partially dried dispersion. Once sufficient water has beenremoved from the alumina sol or dispersion, the partially dried plasticmass may be shaped by any convenient method such as pressing, molding orextrusion and then carefully dried to produce the desired shape such asa rod, pyramid, diamond, cone and the like. Irregularly shaped abrasivegrits are conveniently formed by simply depositing the sol or dispersionin any convenient size of drying vessel such as one in the shape of acake pan and drying, usually at a temperature below the frothingtemperature of the sol or dispersion.

Drying may be accomplished by simply air drying or using any of severalother dewatering methods that are known in the art to remove the freewater of the sol to form a solid. After the solid is dried, it can becrushed or broken by any suitable means, such as a hammer or ball millto form grits or particles. Any method for comminuting the solid can beused and the term "crushing" is used to include all of such methods.

After crushing, the dried grits or particles are then calcined to removeessentially all volatiles. The dry grits are generally heated to atemperature between 400° C. and about 800° C. and held within thistemperature range until the free water and a substantial amount of thebound water is removed, "preferably over 90 weight percent of the totalwater. The calcined grits may then be screened and grits of a particularsize or the entire batch may be used in the next steps.

The metal oxide additive or its precursor, typically a salt of themetal, is added to a liquid vehicle such as water to provide animpregnation mixture or solution, if the oxide or salt dissolves. Theimpregnation mixture is then imbibed or doped into the porous grits ofcalcined alumina. Sufficient impregnation mixture is added to thecalcined alumina grits to provide at least about 0.5% by weight andpreferably about 1 to about 30% by weight metal oxide additive, ascalculated on a fired solids basis. It should be understood that thisconcentration of metal oxide additive will be the goal of theimpregnation process, but the concentration will be the averageconcentration throughout each fired impregnated grit. Impregnationtypically produces impregnated grits with higher concentrations of metaloxide additive at or near the surface than in the interior of theimpregnated structure.

The concentration differential is thought to be as a result of greaterconcentrations of metal ions being at the surface caused by lack ofpenetration of the liquid vehicle into the interior of the porous gritsor by outward migration of the metal ions once impregnated into theinterior as drying occurs. The concentration differential caused bymigration may be reduced by employing a liquid vehicle which is modifiedto have increased viscosity or by reaction to cause precipitation of themetal ions within the interior of the porous grit. Conventionalviscosity modifiers may be used to increase the viscosity. Such increaseof viscosity should not be so great, however, so as to prevent adequatepenetration of the solution. Reaction of some of the impregnationsolutions with NH₄ OH can cause the metal ions to precipitate within theporous grit. Other similar reactants will also be useful. Reactants andviscosity modifiers must be selected so as not to leave a residue oncalcining or firing which will adversely affect these steps or the firedproduct.

The preferred metal precursor is a salt of a volatile anion. Metal saltshaving volatile anions include, for example, the metal nitrate, formate,acetate or the like. The most readily available chemical form of metalis typically the oxide which is easily converted to a salt with avolatile anion by reaction with an excess of concentrated nitric acid toproduce nitrate solution which can conveniently be introduced into theporous alumina body in the desired amount. Metal salts and compoundswhich remain stable and have anions which do not volatilize at least atthe firing temperature of the ceramic material should be avoided sincethey generally do not react with alumina to form the desiredmetal-aluminum oxide reaction product, as will hereinafter be explained.The metal may also be introduced as the oxide, for example, as finelydivided hydrated particles as in a sol. The impregnating solution maycontain other additives, e.g., a nucleating agent to enhance thetransformation of the alumina hydrate to alpha alumina. The nucleatingagent may also be contained in the alumina hydrate dispersion or it maybe solely contained in the dispersion or the impregnating solution.

Impregnation may be accomplished in a single impregnation, i.e., usingonly one impregnation solution a single time or it may be accomplishedin several impregnation steps. That is, more than one impregnationsolution may be applied to a particular porous structure. For example,the same impregnation solution may be applied once, and, after thedrying and calcining steps, applied again to increase the concentrationin the porous structure of the solids being carried in the impregnationsolution. The subsequent impregnation solution may also have a differentconcentration of solids and/or a combination of different solids. Forexample, the first solution may contain one metal salt and the secondsolution may contain a different one.

After impregnation, the impregnated grits are dried and calcined toremove bound volatile materials. Calcining is usually accomplished at atemperature of between about 400°-800° C.

The impregnated calcined grits are then sintered by heating to atemperature between about 1200° C., to about 1650° C. and holding withinthis temperature range until substantially all of the additive metaloxide reacts with alumina under conditions to thereby be converted to areaction product and until substantially all of the remaining alumina isconverted to a fired form, typically alpha alumina. Of course, thelength of time to which the calcined material must be exposed to thesintering temperature to achieve this level of conversion will dependupon various factors but usually will be accomplished within seconds toabout 30 minutes.

Other steps can be included in this process, such as rapidly heating thematerial from the calcining temperature to the sintering temperature,sizing granular material, centrifuging the dispersion to remove sludgewaste, etc. Moreover, this process may be modified by combining two ormore of the individually described steps, if desired.

The firing steps are more fully described in U.S. Pat. No. 4,314,827,the disclosure of which is herein incorporated by reference.

The ceramic materials according to the invention may have a densityvarying from near its theoretical density, e.g., 95% or greater, toabout 75%. The ceramic material may be substantially void free or it maybe characterized by including porosity, typically in the form ofinternal vermicular or equiaxial pores which are for the most part onthe interior of the ceramic with a minor part of the pores extending tothe surface. Porosity is very difficult to measure accurately byconventional porosity measuring techniques because the porosity is a mixof closed pores which do not extend to the surface and open pores whichdo.

The ceramic abrasive grits according to the present invention may beutilized in conventional abrasive products, preferably as a blend withless expensive conventional abrasive grits such as fused alumina,silicon carbide, garnet, fused alumina-zirconia and the like. It mayalso be blended with particulate diluent minerals or materials which arenot noted as abrasives such as glass and the like.

Because of the relatively high cost of the rare earth metal compounds,it is preferred to blend ceramic abrasive grits which contain expensivestarting materials with less expensive abrasive minerals. Such blendingof abrasive grits is known. A preferred method of blending is describedin assignee's U.S. Pat. No. 4,734,104, issued Mar. 29, 1988, involving amethod known as selective mineral substitution wherein the coarseabrasive is removed from an inexpensive abrasive grit charge that is tobe utilized in an abrasive product such as a coated abrasive and issubstituted with coarse mineral of the invention. It is recognized inthat patent application that in any coated abrasive the coarse abrasivegrits are substantially responsible for a major portion of the abradingof a workpiece. By such substitution, the improved abrasive grits of thepresent invention are interposed in an abrasive product between smallergrits of conventional abrasive mineral to permit the improved coarseabrasive grits to do the bulk of the abrading with such product.Aforementioned U.S. Pat. No. 4,734,104 is incorporated herein byreference for its disclosure of this feature.

The ceramic abrasive grits of the present invention are convenientlyhandled and incorporated into various abrasive products according towell-known techniques to make, for example, coated abrasive products,bonded abrasive products, and lofty non-woven abrasive products. Themethods of making such abrasive products are well-known to those skilledin the art. A coated abrasive product includes a backing, for example,formed of fabric (e.g., woven or non-woven fabric such as paper) whichmay be impregnated with a filled binder material, a polymer film such asthat formed of oriented heat-set polypropylene or polyethyleneterephthalate which may be first primed, if needed, with a primingmaterial, or any other conventional backing material. The coatedabrasive also includes a binder material, typically in layers includinga make or maker coat, a size or sizing coat and possibly a supersizecoat. Conventional binder materials include phenolic resins.

It has been found that the addition of a grinding aid over the surfaceof the abrasive grits typically in the supersize coating provides animproved grinding performance when using a coated abrasive productcontaining the ceramic abrasive grits of the present invention. Grindingaids may also be added to the size coat or as particulate material. Thepreferred grinding aid is KBF₄, although other grinding aids are alsobelieved to be useful. Other useful grinding aids include NaCl, sulfur,K₂ TiF₆, polyvinylidene chloride, polyvinyl chloride, cryolite andcombinations and mixtures thereof. The preferred amount of grinding aidis on the order of 50 to 300 g., preferably 80 to 160 g. per squaremeter of coated abrasive product.

Non-woven abrasive products typically include an open porous loftypolymer filament structure having the ceramic abrasive grits distributedthroughout the structure and adherently bonded therein by an adhesivematerial. The method of making such non-woven abrasive products is wellknown.

Bonded abrasive products typically consist of a shaped mass of abrasivegrits held together by an organic or ceramic binder material. The shapedmass is preferably in the form of a grinding wheel. The preferred bindermaterials for the ceramic abrasive grits of the invention are organicbinders. Ceramic or vitrified binders may be used if they are curable attemperatures and under conditions which will not adversely affect theceramic abrasive grits of the present invention.

EXAMPLES

The following examples are illustrative of certain specific embodimentsof this invention; however, these examples are for illustrative purposesonly and are not to be construed as limitations upon the invention. Allparts are by weight, unless otherwise specified.

EXAMPLES 1-32

Room temperature deionized water (2600 ml), 48 g of 16N analyticalreagent grade nitric acid and 800 g alpha aluminum oxide monohydratepowder sold under the trade designation Disperal® were charged into an18.9 liter polyethylene-lined steel vessel. The charge was dispersed athigh speed for five minutes using a Gifford-Wood Homogenizer Mixed(Greeco Corp., Hudson, N.H.). The resulting sol was poured into a 46cm×66 cm×5 cm polyester-lined aluminum tray where it was dried in aforced air oven at 100° C. to a friable solid.

The resultant dried material was crushed using a "Braun" type UDpulverizer having 1.1 mm gap between the steel plates. The crushedmaterial was screened and the 0.125 mm to about 1 mm screen sizematerial was retained and was fed into the end of a calciner which was a23 cm diameter 4.3 meter long stainless steel tube having a 2.9 meterhot zone, the tube being inclined at 2.4 degrees with respect to thehorizontal, and rotating at 7 rpm, to provide residence time therein ofabout 15 minutes. The calciner had a hot zone feed end temperature of350° C. and exit end temperature of 800° C.

The prefired material (100 grams) was added to 300 ml of the rare earthnitrate solution of the concentration given in Table I contained in a500 ml glass filtering flask. An aspirator was used to pull a partialvacuum above the solution which allowed air trapped within the porosityof the grits to escape, and the rare earth nitrate solution tocompletely infiltrate the porosity. The partial vacuum was maintainedfor about one minute, after which the excess nitrate solution wasremoved by filtering the saturated grits over No. 4 filter paper. Thegrits were dried in a forced air oven at 100° C., then fed through arotary calciner as described previously. For multiple impregnations, theprefired material is allowed to cool and is then impregnated again inthe desired solution. Excess solution is removed, the material is driedand prefired again. This process may be repeated as often as necessaryto obtain the desired concentration of rare earth oxide.

The fired product from the calciner was fed into a 1380° C. kiln whichwas a 8.9 cm diameter 1.32 meter long silicon carbide tube inclined at4.4 degrees with respect to the horizontal and having a 76 cm hot zone,rotating at 10.5 rpm, to provide a residence time therein of about 5minutes. The product exited the kiln into room temperature air where itwas collected in a metal container and allowed to cool to roomtemperature.

Table I sets forth details about the preparation of Examples 1-32. Theseexamples involve the use of only one rare earth oxide modifier. Table Ireveals the relative amounts of alumina and modifier and the type ofmodifier. The modifier is shown as the rare earth metal oxide forconvenience but it will typically be present as a complex crystalstructure with the alumina, as previously explained. This crystalstructure is also set forth in Table I.

The concentration of aqueous solution of rare earth metal nitrateemployed in each case is also given. Where two concentrations appear fora particular example, e.g., "23/23" for Example 1, this means that theporous alumina hydrate body was impregnated twice. In Example 1, theconcentration of the rare earth nitrate was the same in eachimpregnation. In Example 6, however, the first impregnation was with a23% solution while the second impregnation was with a 15% solution.

                  TABLE I                                                         ______________________________________                                                                            Rare                                                                          Earth                                                                         Metal                                                                         Nitrate                                                  Rare Earth           Solution                                  Ex.  Al.sub.2 O.sub.3                                                                        Metal Oxide  Crystal Solids                                    No.  %         (Type)  (%)    Structure                                                                             (%)                                     ______________________________________                                         1   74        Dy.sub.2 O.sub.3                                                                      26     Garnet  23/23                                    2   84        Dy.sub.2 O.sub.3                                                                      16     Garnet  23                                       3   91        Dy.sub.2 O.sub.3                                                                      9      Garnet  15                                       4   95        Dy.sub.2 O.sub.3                                                                      5      Garnet  10                                       5   97        Dy.sub.2 O.sub.3                                                                      2      Garnet  5                                        6   83        Dy.sub.2 O.sub.3                                                                      17     Garnet  23/15                                    7   83        Dy.sub.2 O.sub.3                                                                      17     Garnet  23/15                                    8   94        La.sub.2 O.sub.3                                                                      6      Beta Al.sub.2 O.sub.3                                                                 22.6                                     9   94        La.sub.2 O.sub.3                                                                      6      Beta Al.sub.2 O.sub.3                                                                 22.6                                    10   85        Gd.sub.2 O.sub.3                                                                      15     Garnet  22.9                                    11   85        Gd.sub.2 O.sub.3                                                                      15     Garnet  22.9                                    12   89.4      Pr.sub.2 O.sub.3                                                                      10.6   Perovskite                                                                            22.5                                    13   89.6      Pr.sub.2 O.sub.3                                                                      10.4   Perovskite                                                                            20                                      14   90.9      Pr.sub.2 O.sub.3                                                                      9.1    Perovskite                                                                            17.5                                    15   91.8      Pr.sub.2 O.sub.3                                                                      8.2    Perovskite                                                                            15                                      16   94.6      Pr.sub.2 O.sub.3                                                                      5.4    Perovskite                                                                            12.5                                    17   95        Pr.sub.2 O.sub.3                                                                      5      Perovskite                                                                            10                                      18   87        Yb.sub.2 O.sub.3                                                                      13     Perovskite                                                                            22.5                                    19   88.6      Yb.sub.2 O.sub.3                                                                      11.4   Perovskite                                                                            17.5                                                                  and Garnet                                      20   91.3      Yb.sub.2 O.sub.3                                                                      8.7    Perovskite                                                                            12.5                                                                  and Garnet                                      21   87.7      Er.sub.2 O.sub.3                                                                      12.3   Perovskite                                                                            22.5                                                                  and Garnet                                      22   91.4      Er.sub.2 O.sub.3                                                                      8.6    Perovskite                                                                            17.5                                                                  and Garnet                                      23   95        Er.sub.2 O.sub.3                                                                      5      Perovskite                                                                            12.5                                                                  and Garnet                                      24   90.5      Sm.sub.2 O.sub.3                                                                      9.5    Perovskite                                                                            17.4                                    25   92.4      Sm.sub.2 O.sub.3                                                                      7.6    Perovskite                                                                            13.1                                    26   95.7      Sm.sub.2 O.sub.3                                                                      4.3    Perovskite                                                                            8.7                                     27   88.2      Sm.sub.2 O.sub.3                                                                      11.8   Perovskite                                                                            17.4/8.7                                28   89.8      Sm.sub. 2 O.sub.3                                                                     10.2   Perovskite                                                                            18.4                                    29   89.9      La.sub.2 O.sub.3                                                                      10.1   Beta Al.sub.2 O.sub.3                                                                 18.1                                    30   84.3      La.sub.2 O.sub.3                                                                      5.7    Beta Al.sub.2 O.sub.3                                                                 11.3                                    31   91.9      La.sub.2 O.sub.3                                                                      8.1    Beta Al.sub.2 O.sub.3                                                                 15.1                                    32   89.4      La.sub.2 O.sub.3                                                                      10.6   Beta Al.sub.2 O.sub.3                                                                 18.8                                    ______________________________________                                    

Abrasive grits of each of the examples were made into coated abrasiveproducts which were tested for abrasiveness. The coated abrasiveproducts were made according to conventional coated abrasive makingprocedures. The abrasive grits were screened to yield various grainsizes or abrasive grit grades and the desired grade selected for theparticular construction. The abrasive grits were bonded to polyester orvulcanized fiber backings using conventional make, size, and optionallysupersize adhesive resin compositions.

Table II reveals the grit size (grade), the composition of the abrasivegrits, and grinding aid, if used, the total amount of metal removed in agrinding test (Total Cut) and the relative grinding performanceaccording to a "disc test" (unless otherwise specified) as describedbelow when compared to the performance of a standard consisting ofcommercial grade fused alumina-zirconia abrasive grits available underthe trade designation NorZon®. In Table II, the given grade size refersto abrasive grit having an average diameter as follows:

    ______________________________________                                                    Average Diameter                                                  Grade       (micrometers)                                                     ______________________________________                                        36          710                                                               40          600                                                               50          430                                                               ______________________________________                                    

The term "disc" test refers to disc test hereinafter described.

The disc test involved the testing of 17.8 cm diameter abrasive discshaving the following approximate coating weights:

    ______________________________________                                        Grade  Make Resin                                                                              Mineral    Size Resin                                                                            Supersize                                 ______________________________________                                        36     4.2 g     18.8 g     13.6 g  8 g                                       40     4.2 g     18.0 g     13.0 g  8 g                                       50     4.2 g     13.2 g      8.7 g  6 g                                       ______________________________________                                    

Disc Test

The discs were prepared using conventional coated abrasive makingprocedures, conventional 0.76 mm vulcanized fiber backings andconventional calcium carbonate-filled phenolic resin make and sizeresins, without adjusting for mineral density differences. The makeresin was precured for 90 minutes at 88° C. and the size resin precuredfor 90 minutes at 88° C. followed by a final cure of 1002 C. for 10hours. The coating was done using conventional techniques in a one-tripoperation with curing in a forced air oven. The cured discs were firstconventionally flexed to controllably break the hard bonding resins,mounted on a beveled aluminum back-up pad, and used to grind the face ofa 2.5 cm by 18 cm 304 stainless steel workpiece. The disc was driven at5,500 rpm while the portion of the disc overlaying the beveled edge ofthe back-up pad contacted the workpiece at 5.91 kg pressure generating adisc wear path of about 140 cm². Each disc was used to grind a separateworkpiece for one minute each for a total time of 12 minutes each or forsufficient one minute time segments until no more than 5 grams of metalwere removed in any one minute grinding cut. The total cut for each discis reported in Table II. The relative cumulative cut of each of the 12cuts (or for a lesser number of cuts, depending upon performance) foreach disc, using the cumulative cut of a disc made of a control abrasivegrain as 100%, was calculated and is also tabulated in Table II. All ofthe grinding data was generated using the Disc Test unless otherwiseindicated in which case the grinding data was generated using anAbrasive Belt Test as described below.

Abrasive Belt Test

Grade 50 coated abrasive sheets were converted to 7.6 cm×335 cm endlessabrasive belts and tested on a constant load surface grinder, abradingthe 21/2 cm×18.4 cm face of a 304 stainless steel workpiece with 30successive 60 second grinding passes, weighing and cooling after eachpass, employing 25 lb pressure, a 2000 m/min belt speed, and 1.8 m/minworkpiece feed rate.

                  TABLE II                                                        ______________________________________                                        Ex.  Additive   Abrasive Grinding                                                                             Total Cut                                                                             Relative                              No.  (Type)  (%)    Grade  Aid    (Grams) Cut (%)                             ______________________________________                                         1   Dy.sub.2 O.sub.3                                                                      26     40     --      46     107                                  2   Dy.sub.2 O.sub.3                                                                      16     40     --      36      84                                  3   Dy.sub.2 O.sub.3                                                                      9      40     --      22      52                                  4   Dy.sub.2 O.sub.3                                                                      5      40     --      18      41                                  5   Dy.sub.2 O.sub.3                                                                      2      40     --      20      45                                  6   Dy.sub.2 O.sub.3                                                                      17     40     --     105      64                                  7   Dy.sub.2 O.sub.3                                                                      17     40     KBF.sub.4                                                                            354     215                                  8   La.sub.2 O.sub.3                                                                      6      36     --      57      88                                  9   La.sub.2 O.sub.3                                                                      6      36     KBF.sub.4                                                                            127     102                                 10   Gd.sub.2 O.sub.3                                                                      15     36     --     137     110                                 11   Gd.sub.2 O.sub.3                                                                      15     36     KBF.sub.4                                                                            231     186                                 12   Pr.sub.2 O.sub. 3                                                                     10.6   50     KBF.sub.4                                                                             54      88                                 13   Pr.sub.2 O.sub.3                                                                      10.4   50     KBF.sub.4                                                                             46      76                                 14   Pr.sub.2 O.sub.3                                                                      9.1    50     KBF.sub.4                                                                             47      76                                 15   Pr.sub.2 O.sub.3                                                                      8.2    50     KBF.sub.4                                                                             45      74                                 16   Pr.sub.2 O.sub.3                                                                      5.4    50     KBF.sub.4                                                                            118     194                                 17   Pr.sub.2 O.sub.3                                                                      5      50     KBF.sub.4                                                                            147     241                                 18   Yb.sub.2 O.sub.3                                                                      13     50     KBF.sub.4                                                                            125     204                                 19   Yb.sub.2 O.sub.3                                                                      11.4   50     KBF.sub.4                                                                            100     164                                 20   Yb.sub.2 O.sub.3                                                                      8.7    50     KBF.sub.4                                                                             85     140                                 21   Er.sub.2 O.sub.3                                                                      12.3   50     KBF.sub.4                                                                            197     322                                 22   Er.sub.2 O.sub.3                                                                      8.6    50     KBF.sub.4                                                                            152     249                                 23   Er.sub.2 O.sub.3                                                                      5      50     KBF.sub.4                                                                            114     186                                 24   Sm.sub.2 O.sub.3                                                                      9.5    50     KBF.sub.4                                                                            196     182                                 25   Sm.sub.2 O.sub.3                                                                      7.6    50     KBF.sub.4                                                                            232     214                                 26   Sm.sub.2 O.sub.3                                                                      4.3    50     KBF.sub.4                                                                            177     164                                 27   Sm.sub.2 O.sub.3                                                                      11.8   50     KBF.sub.4                                                                             71      71                                 28   Sm.sub.2 O.sub.3                                                                      10.2   50     --      64     --                                  29   La.sub.2 O.sub.3                                                                      10.1   50     --      85     --                                  30   La.sub.2 O.sub.3                                                                      5.7    50     --      510*    84                                 31   La.sub.2 O.sub.3                                                                      8.1    50     --      429*    70*                                32   La.sub.2 O.sub.3                                                                      10.6   50     --      105*    17*                                ______________________________________                                         *Belt Test                                                               

While this invention has been described in terms of specificembodiments, it should be understood that it is capable of furthermodifications. The claims herein are intended to cover those variationswhich one skilled in the art would recognize as the chemical equivalentof what has been described here.

What is claimed is:
 1. An abrasive wheel comprising binder and abrasivegrits, said abrasive grits comprising ceramic abrasive grits beingcomprised of alpha alumina and at least about 0.5% by weight of at leastone modifying metal oxide, the metal of the metal oxide being selectedfrom the group consisting of zirconium, hafnium, cobalt, nickel, zinc,magnesium, yttrium, praseodymium, samarium, ytterbium, neodymium,lanthanum, gadolinium, cerium, dysprosium, erbium, and combinations oftwo or more of such metals, wherein the concentration of said modifyingmetal oxide is greater at or near the surface of said grit than at theinterior of said grit, and wherein said ceramic abrasive grits have ashape selected from the group consisting of rods, pyramids, diamonds,and cones.
 2. The abrasive wheel of claim 1 wherein said ceramicabrasive grits have a rod shape.
 3. The abrasive wheel of claim 1wherein said ceramic abrasive grits comprise modifying metal oxide whichis the oxide of yttrium and one or more metals selected from the groupconsisting of praseodymium, samarium, ytterbium, neodymium, lanthanum,gadolinium, cerium, dysprosium, and erbium.
 4. The abrasive wheel ofclaim 1 wherein said ceramic abrasive grits comprise modifying metaloxide which is an oxide of a metal selected from the group consisting ofzirconium, hafnium, cobalt, nickel, zinc and magnesium.
 5. The abrasivewheel of claim 1 wherein said ceramic abrasive grits comprise modifyingmetal oxide which forms a spinel with alumina.
 6. The abrasive wheel ofclaim 5 wherein said spinel comprises magnesium-aluminum spinel.
 7. Theabrasive wheel of claim 1 wherein said ceramic abrasive grits comprisemodifying metal oxide which forms a garnet with alumina.
 8. The abrasivewheel of claim 1 wherein said ceramic abrasive grits comprise modifyingmetal oxide which forms a perovskite with alumina.
 9. The abrasive wheelof claim 1 wherein said ceramic abrasive grits comprise modifying metaloxide which forms beta alumina with alumina.
 10. The abrasive wheel ofclaim 1 in which the ceramic abrasive grits comprises modifying metaloxide having content is from about 0.5 to about 30% by weight on a firedsolids basis.
 11. The abrasive wheel of claim 1 further including anucleating agent in said ceramic abrasive grits.
 12. The abrasive wheelof claim 1 further including diluent particles selected from the groupconsisting of marble particles and glass particles.
 13. The abrasivewheel of claim 1 wherein said binder is selected from a group consistingof organic binder material, ceramic binder materials and vitrifiedbinder materials.
 14. The abrasive wheel of claim 1 wherein said ceramicabrasive grits are rod shaped and said binder material is an organicbinder.
 15. The abrasive wheel of claim 1 wherein said abrasive gritscomprise said ceramic abrasive grits and other abrasive grits.
 16. Theabrasive wheel of claim 15 wherein said other abrasive grits areselected from the group consisting of fused alumina, silicon carbide,garnet and fused alumina-zirconia.
 17. An abrasive wheel comprisingbinder, diluent particles selected from the group consisting of marbleparticles and glass particles and abrasive grits, said abrasive gritscomprising ceramic abrasive grits being comprised of alpha alumina andat least about 0.5% by weight of at least one modifying metal oxide, themetal of the metal oxide being selected from the group consisting ofzirconium, hafnium, cobalt, nickel, zinc, magnesium, yttrium,praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium,cerium, dysprosium, erbium, and combinations of two or more of suchmetals, wherein the concentration of said modifying metal oxide isgreater at or near the surface of said grit than at the interior of saidgrit.